As is known in the art, a balun is a circuit element often used to convert unbalanced transmission line inputs into one or more balanced transmission line outputs or vice-versa. Baluns operating at relatively low-frequencies (e.g., below 1 GHz) are generally provided using ferrite and air coil transformer technology to achieve high performance and relatively broad bandwidth.
There has, however, been a trend to employ baluns in a wide variety of different types of applications often requiring high-frequency and/or wideband operation. For example, baluns have been included in output stages of delta-sigma modulator direct digital synthesizers, Digital-to-Analog Converters (DACs), Analog-to-Digital Converters (ADCs), differential digital signaling, RF mixers, SAW filters, balanced amplifier circuit configurations, feeding differential antenna elements in array antennas and other antenna feeds, for example, antenna feeds for cognitive radio systems. All of these applications require a balun which operate over a relatively wide bandwidth and which are compatible with integrated circuits and capable of rejecting common mode energy from differential inputs or providing differential outputs lacking common mode energy.
For baluns operating at or above microwave frequencies, it has become increasingly more difficult to fabricate broadband baluns utilizing ferrite and air coil transformers. This has necessitated that other techniques be used. Baluns operating at such high-frequency bands generally are provided using distributed components rather than from ferrite and air coil technology. Such baluns often comprise quarter-wavelength matching elements or transformers having a size determined according to desired operating wavelengths (i.e. operating frequencies) of the balun. One disadvantage of this approach is that the operational frequency bands of such baluns are fundamentally narrow. Moreover, high frequency signals (e.g., microwave and millimeter wave signals) typically rely on single-ended and unbalanced anti-phase signals (i.e. a signal driven with reference to a ground), rather than balanced differential signals. Such single-ended signals may be beneficial in controlling electromagnetic interference. The corresponding structures, however, are not well suited to accommodate balanced differential signals, which are necessarily isolated from ground.
As is also known, three port balun structures and other differential lines typically do not provide good common mode isolation between the differential ports. For many circuit and array antenna applications, this limits system performance. In particular, differential fed, wide band antenna elements frequently excite a common mode excitation at large scan angles (e.g. at scan angles of about 45 degrees or greater) in an E-plane scan plane.
Prior art systems have used resistive common mode isolators implemented at the differential ports of the balun to terminate this excitation and improve overall array performance.
Other wideband antenna array structures which have encountered this common mode problem have developed solutions for mitigating the effect of this common mode excitation. Such solutions involve E-plane walls or shorting vias to place the common mode in cutoff. These solutions accomplish the goal of limiting the common mode, but at the expense of antenna bandwidth and performance. Other approaches for limiting the common mode include material shaping and/or the use of materials having a relatively low relative dielectric constant. Such materials, however, have undesirable properties from a processing and mechanical point of view and thus increase the cost of manufacturing the circuit. Furthermore, in many cases, the use of low relative dielectric constant materials is still not sufficient to eliminate the problems, particularly in those applications requiring relatively large operating bandwidths.
Thus, a significant design challenge is posed when seeking to provide baluns having both high performance characteristics and/or which operate at relatively high frequencies and/or over relatively wide operating frequency bandwidths while at the same time maintaining phase and amplitude balance. As noted above, balun performance may be a limiting factor in the performance of array antennas and in other RF/microwave systems. Additionally, it is desired to make the baluns small, such that one or more baluns can fit within a unit cell of an array antenna to enable the array antenna to achieve desired performance and functionality.
Conventional balun designs used in circuit and array antenna applications at microwave frequencies include so-called Marchand baluns. Marchand baluns, for example, have been realized in planar form utilizing microstrip technology to provide balun configurations which function over fractional bandwidths of between 6:1 and 9:1. Due to dispersions inherent in implementing microstrip circuits, the performance of such microstrip baluns degrades at higher operating frequencies, reducing the upper frequency band limit and the overall bandwidth of these circuits. This leads to amplitude and phase imbalance at the higher frequencies.
Double Y baluns and similar configurations are also known and have been used to realize baluns having fractional bandwidths of 9:1 and greater (18:1 in some cases). However, such baluns typically suffer from one of two shortcomings depending upon the design. One shortcoming is they often require more space than is available within a unit cell once a transition to an appropriate transmission line mode is included. While compact designs are available in coplanar form, many systems (e.g. array antennas) require microstrip or stripline interfaces. Thus, another shortcoming is that baluns implemented in coplanar form require transitions from coplanar to microstrip and other transmission line implementations and such transitions are prohibitive in a small space due to the presence of a strong coplanar moding. Strong coplanar moding possesses multiple fundamentally different modes that require complex transitions to match with microstrip and stripline modes without the creation of resonances that degrade performance.
It would, therefore, be desirable to provide a balun having good performance characteristics at RF and microwave frequencies and/or over wide operating bandwidths and which are appropriate for use in feeding differential antenna elements in array antennas, balanced amplifier circuits and other applications.